1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly, to an apparatus and method for controlling an all-optical gain and gain flattening.
2. Description of the Related Art
A general erbium-doped fiber amplifier (EDFA) has a constant gain regardless of the intensity of an input signal in a range where the intensity of the input signal is low. However, if the intensity of the input signal is greater than a specific value, the gain of an optical amplifier is inversely proportional to the intensity of the input signal. Furthermore, this gain level of the optical amplifier varies according to the input signal wavelength and so also gain flattening intended in order to give the same gain to all different wavelength signals is broken due to input signal power variation.
An intelligent function for compensating for the variation of gain or gain flattening according to the variation of an input condition of wavelength division multiplexing (WDM) signals is required of the optical amplifier.
To satisfy this function, an electronic control method and an all-optical method have been suggested. Similar to an automatic gain control (AGC) method in an electrical circuit, the electronic control method is a method of adjusting the intensity of pump energy by sensing the intensity of an input optical signal or the intensity of an output optical signal in a receiver, calculating the sensing result, and outputting the calculation result to an optical amplifier.
Compared to the electronic control method, the all-optical method is relatively simple and has a high gain control speed using friendly optical component technology. However, since a considerable amount of optical energy of an optical amplifier is used not for an input signal amplification but for a lasing light for an all-optical gain controlled (AOGC) function, the all-optical method has a low gain compared to a case where the AOGC function is not used and has a limitation in being able to keep a widen a dynamic range in which the AOGC function is maintained in an input signal intensity domain. Additionally this low gain results in worse noise figure.
Since a general AOGC method has a narrow dynamic range, a general AOGC method cannot be applied to an input signal having high intensity, e.g., an input signal using WDM channels in a wide wavelength band, which use all of the C-band (1530 nm-1560 nm). Thus, although a general AOGC method basically has a characteristic of a fixed gain slope, a gain and gain flattening cannot be automatically controlled together due to the low gain and the narrow dynamic range of the general AOGC method.